The present invention relates to a new process for the production of castings with well-defined reproduction of detail and a high accuracy of dimensional tolerance using a new molding process. The new molding process is especially suitable for high-melting metals and alloys.
Molding processes for high-melting metals are known, as well as for precision casting. The best known precision casting molding process is the lost wax process, by which the pattern is melted out for the preparation of the mold. Other processes use synthetic materials or frozen mercury as the material for the pattern. A disadvantage of this molding process is that in order to obtain a precision casting, a number of auxilliary agents and additional measures are required, so that still today in many branches of industry precision casting is considered unsatisfactory as a means for industrial mass production, in particular as too cost intensive and requiring too much labor. Patterns for high melting ferrous and non-ferrous metals must be covered with a fine coating of high-grade, largely silicon bonded ceramic synthetic, because only in that way is a smooth surface produced so that the finest contours can be reproduced. Only then can the molding material, which provides the mold with the stability necessary to withstand the casting pressure, be poured into the molding box.
U.S. Pat. No. 3,825,058 discloses a process for preparing a mold by a vacuum sealed molding process in which a particulate material, for example, sand, is placed inside a molding box, subjected to a vacuum and compressed to form a mold. The mold has a casting cavity which is defined by a shield member, impermeable to gas, made of synthetic material (plastic) or metal foil. In order to prevent breakdown of the mold during pouring of a molten metal, a tubular member forming a passage serving as a communication means between the mold cavity and the atmosphere is connected to the uppermost portion of the shield member so that the atmospheric pressure can be imparted to the cavity. In this way, the vacuum in the space between the particles is certain.
U.S. Pat. No. 3,923,525 discloses foundry molding materials (foundry mixes) which contain an aggregate, for example, sand, and up to about 10 weight % in relation to the foundry molding material of a binder system, with the binder system consisting of 60 to 95 weight % aluminum phosphate containing boron, 5 to 40 weight % alkaline earth oxide or alkaline earth hydroxide, and 15 to 50 weight % water. The foundry mold materials according to the patent are processed to foundry shapes or casting molds in the following steps:
(1) introducing the foundry mix into a mold or pattern to thereby obtain a green foundry shape;
(2) allowing the green foundry shape to remain in the mold or pattern for a time at least sufficient for the shape to obtain a minimum stripping strength (i.e., become selfsupporting); and
(3) thereafter removing the shape from the mold or pattern and allowing it to cure at room temperature, thereby obtaining a hard, solid, cured foundry shape.
The patent states that the binder systems cure at room temperature. The patent does not contain any disclosure of calcining (burning) the binder at a temperature of at least 250.degree. C. after removal of water.
The patent states that it it recognized that the use of the binder system for the foundry shapes intended in the patent is quite distinct from preparing other shaped articles such as ceramics and shapes for precision casting. In particular, the patent states that a suitable binder for shapes for precision casting will not necessarily be applicable as the binder in foundry shapes of the patent.